1. The Field of the Invention
The present invention relates to the field of trucks for railroad cars, and in particular, to steerable trucks for railroad cars.
2. The Prior Art
The wheels which are used on railroad trucks are, almost universally, formed with conical tapered profiles. That is, the diameters of the wheels decrease, with the portions having the smallest diameter facing outwardly, relative to the railroad car. In addition, rims, having overall diameters substantially greater than the largest diameter portion of the tapered wheel surface, are located at the innermost portions of the wheels, and placed on the truck axles, such that the distance between the rims of the wheels on an axle (collectively, "wheel set") is slightly less than the distance between the inside edges of the rails.
In prior art conventional railroad trucks, the axles would be fixed relative to the truck. Typically, there would be provided two trucks situated adjacent the ends of a railroad car. Each truck is connected to the railroad car by a short, very large diameter (typically 14 or 16 inches) cylindrical post extending downwardly from the carbody, which is received by a "bowl" mounted generally centrally relative to the truck. The center post in such a typical prior art configuration would typically have been configured to permit a certain amount of pivoting of the truck, relative to the railroad car body. As a practical matter, the large frictional forces generated by the large surface contact area between the post and the bowl, and the tremendous weight of the carbody, means that the amount of pivoting will be small, and the resistance to pivoting will be great.
As a railroad car having such prior art trucks would enter a curve, the difference in the radii of curvature of the arcs being followed by the "inside" and "outside" wheels would force the axles of the wheel set to adjust by yawing. The natural tendency of a single axle wheel set, in a curve, is to assume a posture is which the axle "points" to the center of curvature of the curve. This movement of a single axle may be referred to as "going radial". In a prior art two wheel set truck with fixed axles, the axles would not be free to assume this described posture independently of one another, and the truck as a whole would be forced to rotate about the center of the truck. This condition would create high stresses on the wheel sets and the truck, increased wear on the truck components, and increased rolling friction, resulting in increased fuel consumption as a result of the additional energy which had to be expended to keep the railroad cars moving.
An additional drawback to prior art truck configurations was a result of the flexibility which permitted the truck to pivot about the center post during turns. Since the wheel profiles were (and are) conical, during straight line travel, there would be (and is) a tendency of the wheels of a single wheel set to alternately oscillate on their respective rails between "high" and "low" positions on the respective wheel profiles. This oscillation would translate into a force tending to cause the truck, as a whole, to pivot about the center post, thus causing the truck, and the car body, to describe a sinusoidal path along the track. This phenomenon is commonly called "hunting". This instability starts at low speeds and can lead to unacceptable lateral wheel force, acceleration and frequency, unless constrained. The instability transfers rolling energy into undesirable lateral energy which could create rolling resistance, lading and car damage, and wheel and track wear.
A typical prior art truck configuration would comprise two longitudinally extending (i.e., track-wise extending) side frames, with a transversely extending bolster attached to the side frames (the "three-piece truck"). The axles of the wheel sets would be mounted fore and aft of the bolster, with the axle ends being generally fixed relative to the side frames.
Even though nominally rigidly constructed, such a truck configuration would, under sufficient loading (such as during curves), deform. Typically, this deformation would take the form of the side frames, bolster and wheel sets skewing relative to one another to form a parallelogram, as the forces exerted on the wheels push the axles to seek yawed positions through the curve. Such parallelogramming is believed to be a common cause of railroad car derailment at low speed in curves.
Accordingly, it can be seen that making trucks rigid and mounting them rigidly to car bodies (in an effort to eliminate hunting), and providing truck pivoting and/or flexibility, to permit truck or axle yawing or steering in curves, can and have created a design impasse for the creation of an effective three piece truck.
Numerous attempts have been made to produce trucks which satisfy the requirements for efficient rolling during both straight runs and curves. Such attempts have included the provision of resilient or elastic members in the side frames and/or bolsters, pivot-mounted axles and side frames with damping apparatus like shock absorbers, and various forms of cross-bracing and the like. Such prior art configurations typically have resulted in truck structures which are costly, heavy, and/or overly complex and prone to failure or requiring extensive maintenance and replacement of components.
It is an object of the present invention to provide a truck which is configured to permit and accommodate the axles' natural tendency to go radial, so as to permit more efficient and less damaging rolling action in curves.
It is another object of the present invention to provide a truck which is configured to have a reduced tendency to hunt, during straight run travel, so as to reduce the damage and rolling inefficiencies associated with hunting.
It is a further object of the present invention to provide a truck having the characteristics sought, which has a simplified and efficient configuration.
These and other objects of the present invention will become apparent in view of the present specification, claims and drawings.